Simple shading problem

Hi,
I'm trying to do a simple cloth simulation based on the Opengl SuperBible.
We consider that we have vertices that are connected to each other.
For each vertice, we only consider perturbation due to 4 adjacents points.
In my code below I first try to find the index of each vertex being processed then its 4 neighbours that I put in connection array.

Than I simply apply physics laws.
The problem is that I have warnings :

0(31) : warning C7050: "connection[0]" might be used before being initialized
0(31) : warning C7050: "connection[1]" might be used before being initialized
0(31) : warning C7050: "connection[2]" might be used before being initialized
0(31) : warning C7050: "connection[3]" might be used before being initialized

I don't understand why the neighbours are not being initialized ?
And obviously the rendering result is not what I expected.

Well, imagine in your shader is invoked on gl_VertexID == 0, where ix == 0 and iy ==0. Then only connection[0] and connection[1] will be assigned a value, and [2] and [3] will be uninitialized. Other vertex shader invocations will not fill in the other connection elements for you, nor do values you assigned in previous invocations affect the next invocation of the shader on a vertex.